Explanation

A higher string stiffness means more control, less power, and less comfort. Stiffer strings exhibit higher peak impact tension, higher peak force, less string deflection, and shorter dwell time (time on the strings). This leads to less power because the higher force crushes the ball more and loses energy. It leads to less comfort because the higher force for a shorter time creates greater shock to the hand and arm. And it creates more control because less time on the strings means less time for the force of ball impact to twist and turn the racquet's aim and also less time for the ball to take a ride on a racquet swing that is constantly changing the direction of its aim as it sweeps through both its horizontal and vertical arcs.

Just the opposite is true for softer string — there is more power due to less ball deformation, more comfort because lower forces are spread out over more time, and less control because longer dwell time leads to more arc-sweep during the bounce off the racquet, leading to more angular spraying.